Let me see if I can put it all together in one post for you:
If you are looking to improve the sound, first off you should be ripping files in either a lossless or an exact-duplicate file convention. EAC, WAV, and or Apple Lossless are a few. Ripping files, or downloading them as compressed versions such as MP3 will not result in very good reproduction in my experience. No matter how good the components you stick in between those files and your speakers they will not be able to accurately replace the information that is lost in compression.
The critical stage of what you are considering is turning the digital bits into an Analogue signal. This is what a DAC does. You can do this a few different ways, the two most effective of which are going with some USB interface like a Waveterminal U24 (my choice, though no longer available) which takes the USB output from the computer, clocks it, and converts it to an S/PDIF signal which can then be fed to a conventional DAC of your choice. This can be a superb solution, depending upon the interface device you choose, as can be going with a more direct solution of a USB DAC like the Apogee or Wavelength or Burwen (there are others and their numbers are increasing). In those case you do not need the interface as you are going straight from the computer into the DAC. From the DAC you can go either directly to a powered (amplified) pair of speakers - there are many professional monitor speakers that have amps that do a good job, or there are inexpensive alternatives like the Swan M200's which sound pretty darn good in spite of their $200 price tag. I have no doubt they will best the HK speakers that came with your Dell. You can also feed the DAC into your home stereo system or HT and get the full-on fireworks of going that route. Pick your poison.
The Apple Airport Express, and similar wireless devices, allow you to broadcast a signal from your computer to some other area in your home and either act as a standalone DAC, of which their actual performance leaves a whole lot to be desired, or, in the case of the AE, can also stream digital via a mini-Toslink cable into a DAC of choice and on to either a system or powered speakers. I prefer the S/PDIF connection so I don't use my AE much, though it can be handy if you prefer to have the computer in a different room than your system.
One other piece of advice: If you decide to build up a music library in lossless or uncompressed files, purchase an external hard disk just to store your music. That way it is portable and will not clog your hard drive on your computer. A 250 gig hard drive costs about $200 and will store about 350-400 CD's in Apple Lossless.